An evaluation of isotherms in the description of competitive adsorption equilibria

Since adsorption isotherm equations are widely used in the industry to model adsorption on substances like activated carbon, an investigation was done to determine the applicability of three of these equations, viz. the multi-component Freundlich, multi-component Langmuir and the isotherm equation proposed by Fritz and Schluender. Based on statistical thermodynamic principles a discrete adsorption model was recently proposed. This model was used to create sets of binary adsorption data with some non-ideal characteristics. These data sets were used to evaluate the isotherm equations. It was found that the Langmuir isotherm gave the best fits, except where the visible adsorption energy changes with the surface loading or where selective adsorption occurs. Under these conditions, the Freundlich equation or that proposed by Fritz and Schluender should be used. It is recommended that these equations should be used to model low coverage-low concentration regions, while the high coverage-high concentration regions should be modelled using the Langmuir isotherm if possible. Some recommendations are made in connection with the procedure to estimate parameters for these isotherm equations.